Carbon black is an engineered, particulate elemental carbon found in countless items used on a daily basis. It is an essential ingredient in, for example, tires and other mechanical rubber goods, improving their strength, durability, and overall performance. It also is used as a pigment in, for example, printing inks, paints, and plastics.
Carbon blacks for rubber applications are typically identified by a four-character “N” or “S” number, e.g., NXXX or SXXX. The category (grade) is determined by ASTM D1765. The first character of the category gives some indication of the influence of the carbon black on the rate of cure of a typical rubber formulation containing the black. The second character gives information on the average surface area of the carbon black. Blacks with the same second character are grouped into a series ending in “00,” e.g., N200 series. The last two characters are assigned arbitrarily. Iodine absorption number (Iodine no.) (ASTM D1510, ISO 1304) has been the primary indication of surface area for defining different grades. Nitrogen surface area (NSA, ASTM D6556) and statistical thickness surface area (STSA, ASTM D6556) are now used more frequently for surface area. N-dibutyl phthalate absorption (DBPA) (ASTM D2414, ISO 4656/1)(now oil absorption number, OAN, ASTM D2414) has been the primary structure indicator in distinguishing different carbon black grades.
The physical characteristics (morphology) of carbon black, such as particle size and structure, affect various processing characteristics and various performance properties of end products, e.g., tires, such as tire treadwear, rolling resistance, heat buildup, and tear resistance. Accordingly, different grade carbon blacks are used in different polymeric formulations depending on the specific service requirements of the tires. Various grades of carbon black are also used in various parts of a tire, for example, N100, N200, and N300 series blacks are often used in treads, while N300, N500, N600, and N700 series blacks are often found in sidewalls and carcasses.
Morphological characteristics of carbon black include, for example, particle size/fineness, surface area, aggregate size/structure, aggregate size distribution, and aggregate shape.
Particle size is a measurement of diameter of the primary particles of carbon black. These roughly spherical particles of carbon black have an average diameter in the nanometers range. Particle size can be measured directly via electron microscopy or by indirect surface area measurement. Average particle size is an important factor that determines relative color strength of a carbon black and dispersibility. At equal structure, smaller particle size imparts stronger color and increased difficulty of dispersion. Fineness is a measure of the particle size.
Surface area of carbon black is a function of particle size and porosity. Surface area is measured by gas and liquid phase adsorption techniques and depends on the amount of adsorbent required to form a surface monolayer. Nitrogen surface area (NSA, ASTM D6556) and statistical thickness surface area (STSA, ASTM D6556) are better measures than iodine adsorption number (Iodine no., ASTM D1510) of the true surface area, since they are less influenced by the chemical composition of the carbon black surface. These tests use liquid nitrogen and are based on the original Brunauer, Emmett, and Teller (BET) method, but use a multi-point measurement to exclude the adsorption in the micropores. In a final application, surface area reflects the area accessible to rubber molecules per unit weight of carbon black. High surface area is associated with a high level of reinforcement, but at the expense of more difficult dispersion, processing, and increased hysteresis.
Carbon black particles coalesce to form larger clusters, aggregates, which are the dispersible units of carbon black. Aggregate size is controlled in the reactor. Measurement of aggregate structure may be obtained from electron microscopy or oil absorption. Grades with relatively large aggregates are high structure grades which are bulkier, have more void space, and high oil absorptions at given surface areas. The carbon black structure is determined by the shape and size of the carbon black aggregates. High structure carbon black increases rubber compound viscosity, modulus, and conductivity. High structure also reduces die swell, loading capacity, and improves dispersibility. Lower structure blacks give higher elongation, and increased carbon black loading reduces the elongation. If all other features of a carbon black are kept constant, narrow aggregate size distribution increases difficulty of carbon black dispersion and lowers resilience.
Aggregate size distribution (ASD) is a measure of the distribution of the size of carbon black aggregates and has been recognized as one factor important in the reinforcing ability of rubber. Donnet, et al., “Carbon Black Science and Technology,” 2nd ed., Marcel Dekker, Inc. New York (1993), pp. 289–347; Jones, “ASTM Committee D24: Keeping the Rubber Industry in the Black,” Standardization News (August 1992; updated Melsom, January 1998) http://www.astm.org/COMMIT/CUSTOM1/D24.htm. Broad ASD carbon black shows a tendency to decrease the rolling resistance of tire tread. You, et al., “A New Characterization method of Tread Carbon Black by Statistical Regression Treatment,” DC Chemical Co. Ltd, (Korea) http://www.dcchem.co.kr/english/product/p petr/image/carbon%20black att2.pdf. A broad aggregate size distribution will provide a faster carbon black incorporation and improved carbon black dispersibility in a polymeric (e.g., rubber) matrix.
Surface chemistry is a measure of chemisorbed species on the carbon black surface. These organic functional groups can enhance performance of blacks in certain applications.
Processing characteristics of the black and the physical characteristics of the end product, such as cured rubber, are often measured (in addition to characteristics of the carbon black itself) to compare the relative effects of various carbon blacks for a given polymeric test formulation. Processing characteristics include, for example, mixing energy and black incorporation time. End product characteristics include, for example, dispersion index, tear, tensile strength, Mooney viscosity, modulus, DIN abrasion, fatigue, and rebound.
Black incorporation time (BIT) is the time required to incorporate carbon black into a particular polymeric formulation. When carbon black is mixed with rubber, the first step is penetration of rubber into void space, replacing the trapped air and eliminating loose black. This step is called carbon black incorporation. The time required to fill all the voids with rubber is referred to as black incorporation time. A short black incorporation time may reduce actual mixing time and increase mixing equipment throughput.
Dispersion index (DI) is a measurement of the dispersion of the carbon black in a polymeric formulation/cured rubber. Following carbon black incorporation, the aggregates are separated from each other and are dispersed throughout the rubber. The state of dispersion of the carbon black is usually measured by carbon black dispersion index. A poor level of carbon black dispersion may cause premature failure of a final rubber product and less favorable ultimate properties, such as fatigue life, tear strength, and tread wear.
Current commercial grade N200 series carbon blacks, such as N234 and N299, can provide good properties in end products, such as tensile, fatigue, and dynamic properties in a rubber composition, if they are properly dispersed in the rubber matrix. However, the dispersion level of these finer conventional grade carbon blacks can differ depending on the rubber formulation and mixing parameters employed. The performance of a rubber composition with a good carbon black dispersion is superior to the same rubber composition with the same carbon black poorly dispersed. Coarser grades such as N300 (and higher numbered) blacks are more easily dispersed but their end product reinforcement characteristics are not as good as the finer blacks, if both are properly dispersed.
Thus, a balancing act between the carbon black grade, rubber formulation (including, e.g., added dispersants), and mixing conditions/time (e.g., greater time and mixing energy for greater dispersion) is required from the rubber compounder. From the standpoint of end product performance, predictability, and operating costs (e.g., energy input and throughput), it is very desirable to be able to engineer carbon blacks to simultaneously provide all of the desired dispersion and performance characteristics.